The present invention generally relates to a device for compensating the baseline drift of a chromatographic separating column during a temperature and/or flow program.
Mixtures are separated by chromatography (gas or liquid chromatography). A separating column contains a separating substance which interacts more or less intensely with the individual constituents of a mixture being examined. In many cases, the separating substance is a liquid in a stationary phase, in which the constituents of the mixture being examined are more or less soluble. A carrier medium (carrier gas or carrier liquid) is conducted through the separating column with a certain flow rate. The mixture to be examined is supplied to the inlet of the separating column and transported by the carrier medium through the separating column. Therein a constituent which, for example, is easily soluble in a liquid stationary phase propagates through the separating column more slowly than a constituent which is less soluble or more volatilized in this stationary phase. Therefore, a sample mixture supplied as a "plug" to the inlet of the separating column separates on its way through the separating column into the individual constituents thereof. The individual constituents propagate through the separating column at different speeds and elute consecutively at the outlet of the separating column. A detector, which responds to the emerging constituents of the sample and supplies corresponding signals, is provided at the outlet of the column. The temporal behavior of the detector signal, in the form of a chromatogram, shows a sequence of bands or peaks, each of which corresponds to a constituent of the mixture.
Certain constituents of a sample propagate only very slowly through the separating column in comparison with other constituents. Therefore, when working at a constant temperature and flow rate, which ensures a proper separation of the easily volatilized constituents, it takes a very long time until the not so easily volatilized constituents appear. As a result, the time of analysis is prolonged. Furthermore, when the time of analysis is prolonged, the bands or peaks are undesirably widened. Therefore, in order to avoid these disadvantages, it is known to vary the temperature and possibly the flow rate according to a predetermined program. When the temperature of the separating column is increased with time according to a certain program, the less easily volatilized constituents are more quickly driven out of the separating column after the easily volatilized constituents have exited. An increase of the flow rate as a function of time has a similar effect.
The detector also supplies a signal when no sample is supplied to the separating column. This signal is constant with constant temperature and constant flow rate. In recording of the chromatogram, this signal results in a straight, horizontal baseline, to which the peaks can be referenced. With a temperture or flow rate program, however, the "baseline signal" varies with the variation of the temperature of the flow rate. This baseline signal variation has to be taken into account when evaluating the chromatogram. For this purpose it is known to provide a second similar reference separating column in addition to the "active" separating column used for separating the mixture. The reference separating column is operated without a sample mixture, but otherwise under identical conditions as the active separating column. The detector signal obtained from the outlet of the active separating column is corrected by the detector signal from the outlet of the reference separating column. However, such an arrangement is expensive. Two separating columns identically formed have to be provided with detectors and placed in an oven correspondingly dimensioned. Furthermore, the exact corresponding formation of the separating columns and the exact corresponding, programmed conditions of operation are necessary for accurate baseline compensation. Failure to meet this requirement can lead to measuring and identification errors.
Furthermore, it is known to store the profile of the baseline during a test run without the sample, but with the predetermined temperature or flow rate program. During a subsequent measuring run, the stored value of the baseline is subtracted from the respective measuring signal. This approach requires a rather high capacity memory. Furthermore, each time the progrm is changed, the baseline has to be recorded anew and stored.